Prosecutors asked a Seoul court on Monday to sentence a former chief of South Korea's intelligence agency to four years in jail over an election-meddling scandal.

In a retrial against ex-National Intelligence Agency chief Won Sei-hoon, the prosecution also demanded a four-year disqualification on the defendant.

Won was indicted in 2013 on charges of interfering in the 2012 presidential election by instructing his subordinates to post political comments online in a bid to sway public opinion in favor of then ruling party contender Park Geun-hye.

Former NIS chief Won Sei-hoon enters the courthouse for a retrial on his election-meddling scandal in Seoul on July 24, 2017. (Yonhap)

The 66-year-old was sentenced to three years in prison by the Seoul High Court in early 2015. But the Supreme Court sent back his case to the lower court in July, saying that the evidence that led to his conviction cannot be seen as valid.

The prosecution argued that Won was guilty based on the attached email files retrieved from an NIS officer's computer that contained hundreds of Twitter IDs and directions on writing online comments.

But the top court overruled the claim citing there was no evidence to substantiate the documents were written upon Won's order or that he was actually involved in the operations. Won headed the NIS from 2009 to 2013 under the conservative Lee Myung-bak administration.

On Monday, the court accepted a fresh set of evidence submitted by the prosecution that includes an audio recording of an NIS meeting. According to the prosecution, the audio tape has Won mentioning the importance of "psychological warfare on the public."

Won initially received a suspended sentence in 2014 for intervening in politics in breach of the NIS law. But he was additionally convicted of willful negligence and violation of election law in an appeal that had immediately put him behind bars.

He served about eight months before he was released on bail in October 2015.

"I tried hard to stay neutral from politics and elections," Won said during the final hearing, denying the allegations raised against him.